The present invention generally pertains to estimating a given parameter, such as the angle of arrival (AOA) of a received target signal, and is particularly directed to making a preferred estimate of the given parameter from data points representing separate estimates of the given parameter.
A computer in a radio frequency (RF) signal direction finding system processes samples of a target signal received by an array of antenna elements to estimate the AOA for the received target signal. In order to estimate the AOA for target signal bursts that are received over short durations, the computer in the direction finding system must be able estimate the AOA by processing very-short-duration samples of the received target signal. If the target signal bursts are of a longer duration, then multiple AOA's are estimated by the computer during each burst of the received target signal. These multiple estimates of the AOA may vary due to (a) poor signal-to-nose ratio (SNR) of the received target signal, (b) antenna limitations, and/or (c) multiple target signals being received simultaneously within the tuned bandwidth of the direction finding system at different AOA's from different targets.
Poor SNR can result from increased background noise in the environment, the range to the target, the movement of the target's transmitting antenna, an obstacle moving in front of the target, or a signal from a wideband transmitter that causes the apparent background noise floor to increase. The SNR can also be affected by interference due to receipt of narrowband interfering received signals within the tuned bandwidth of the direction finding system. This interference can affect the sensitivity of the direction finding system and thereby affects the SNR of the target signal. This interference also adds an unknown component to the samples that are processed by the computer to estimate the AOA.
Antenna limitations, such as the spacing of the antenna elements relative to the wavelength (λ) of the frequency of the target signal, can cause a computer to estimate multiple possible AOAs. The computer of the direction finding system processes samples of the target signal obtained from different antenna elements to estimate the AOA of the target signal by determining the relative phase differences of the samples obtained from the different antenna elements and estimating the AOA in accordance with the relative phase difference that maximizes the energy for the different samples. The AOA of the target signal affects the relative phase of the antenna element at each of the individual antenna elements and thereby affects the relative energy for the received target signal samples obtained from the different antenna elements.
Consider the case of a 4-element circular array of antenna elements, wherein the elements are equally spaced π/2 apart around a circle of diameter D. When the wavelength λ of the target signal becomes equal to or less than the diameter D of the circular array of antenna elements, it becomes difficult to estimate the AOA of target signal.
Now consider the scenario that occurs when the target signal is arriving at an angle that is along the axis of the antenna elements (i.e. 0-degrees, +/−90-degrees, and +/−180-degrees) and the wavelength λ is equal to the diameter D. Assuming that an incoming signal arriving at 0-degrees hits the antenna as a straight wave front, the difference in the phase-dependent energy of the samples of the target signal received by opposing antenna elements is zero, since the elements disposed at 0-degrees and 180-degrees in relation to the AOA are sampling the incoming wave exactly λ apart and the two elements at 90-degrees and −90-degrees in relation to the AOA are sampling at λ/2. In this situation, the computer of the direction finding system determines that there are multiple local maxima. The multiple local maxima present an ambiguity that is due to antenna limitations.
To minimize computation time, the AOA-estimation routine performed by the computer includes a two-stage peak search that could be affected by these local maxima, especially in low SNR conditions. The two-stage peak search is accomplished by first performing a coarse angle search over the whole range of −π to π and selecting the maximum value. Next, a range of angles is selected around the angle corresponding to the selected maximum value and a search is performed to find the angle corresponding to the peak of the maxima. Adding in the effects of noise, in situation discussed above in which the computer determines that there are multiple local maxima, the computer could ultimately estimate that there are multiple different AOAs.
In some communication environments, it is common to receive target signal from two or more targets within the same frequency bandwidth. For a direction finding system that is tuned to monitor this bandwidth, different AOAs will be estimated for target signals received from the different targets. It becomes important for the system to be able to differentiate the targets and their corresponding AOAs to simplify further processing or interpretation of the received target signals.
In scenarios where the wavelength λ is approaching the diameter D of the circular antenna array or the angle or arrival is not along the axis of the antenna, a data fusion algorithm can be used to reduce or eliminate the antenna-limitation effects.
There are various approaches to fusing AOA estimates, such as averaging. Simple averaging of the AOA estimates reduces the number of data points and lessens the effect of the erroneous results, but still allows these estimates to affect the final AOA estimate, since each AOA estimate affects the final estimate equally.
However, merely averaging the AOA estimates does not differentiate between target signals arriving from multiple targets. In the worst case, averaging the AOA estimates for target signals simultaneously received from multiple targets produces an incorrect AOA estimate for any target.